West Nishnabotna Aquifer (Overview)
The West Nishnabotna aquifer consists of alluvial deposits and some terrace deposits associated with the West Nishnabotna River. The West Nishnabotna aquifer supplied urban and rural water users with between 4.2 million gallons of water per day (mgd) to 5.7 mgd from 2004 through 2009. In 2009, an intensive one-year investigation was initiated to provide a comprehensive assessment of groundwater resources in the West Nishnabotna alluvial aquifer.
As part of this investigation, the hydrologic characteristics of the West Nishnabotna alluvial aquifer were examined. Ten (10) aquifer pump and recovery tests were utilized to calculate the hydraulic conductivity and storage coefficient of the aquifer. Twenty (20) observation wells were used to evaluate water levels and groundwater flow in West Nishnabotna alluvial aquifer. Eleven (11) wells had water level data collected over a span of time and were used to calibrate the aquifer model.
With this information, a three layered groundwater flow model of the West Nishnabotna alluvial aquifer was created and calibrated. Layer 1 consists of silt and silty-clay alluvial deposits. Layer 2 is the sands and gravels of the West Nishnabotna alluvial aquifer. Layer 3 consists of tills and shales below the West Nishnabotna alluvial aquifer. The model was created using Visual MODFLOW version 2010.1.
From this regional model, more detailed models of the West Nishnabotna aquifer were created for the four major pumping regions (Figure 1). These models allowed closer examination of local issues, such as quantifying the amount of groundwater that can be pumped without adversely affecting nearby wells.
The model results found a strong interaction between the West Nishnabotna River and the West Nishnabotna alluvial aquifer. On average, approximately 50% of the groundwater withdrawn from the aquifer is originates, or is recharged, from the West Nishnabotna River. The recharge from the river, or induced recharge, provides security for most users of the West Nishnabotna aquifer. As long as water flows in the river, induced recharge will provide water to most wells during droughts and dry periods.
The amount of induced recharge a well can receive is partially dependent on the proximity of the well to the river. Figure 2 shows the percentage of induced recharge each month for the four major pumping regions. Wells in the Manning area are located a few hundred feet from the West Nishnabotna River and average 67% induced recharge. Conversely, wells in the City of Oakland region are located from 0.5 to 1 mile away from the river and only average 5% induced recharge.
Figure 2. Percent induced recharge (river leakage) for the City of Manning/Ag Processing/West Central Iowa Rural Water Association (MAPWC), Harlan, Regional Water, and Oakland local scale models.
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